The internal cavities of the body are lined by epithelial cells which are highly polarized, and many of the functions of epithelia, such as the selective vectorial transport of water, ions and nutrients, critically depend on this polarization. The maintenance of the apical surface as a discrete domain of the plasma membrane that does not exchange membrane proteins with the basolateral domain is essential for proper epithelial function. This research will determine the mechanisms involved in the development and maintenance of this polarized organization of the epithelial plasma membrane as well as the polarized distribution of cytoplasmic organelles in the embryonic epithelium of the amphibian blastoderm. Several possible mechanisms will be investigated: Freeze-fracture electron microscopy will reveal any tight junction protein strands still present in the dissociated blastomere which might serve as barriers to protein diffusion. The cytochalasins will be used to clarify the role of cytoskeletal microfilaments in the maintenance of membrane and cytoplasmic polarity. The recently discovered transcellular blastomere ion current will be studied to determine if it is essential for cell polarization. By blocking this natural current or by imposing small electrical fields across the blastomere, we can determine if this ion current plays a casual role in either the development or maintenance of blastomere polarization.